Completion systems are well known in the art of well production, and can take many varied forms. Well completions typically have as common elements: a casing cemented in the well extending from a surface wellhead to the producing formation; a production tubing located concentrically inside the casing; and one or more well known devices, such as packers, that block, pack off, and seal the annulus formed between the casing and the production tubing, generally by means of a resilient sealing element.
Placement of the packer in this way directs the hydrocarbons from the producing formation into the production tubing and to the earth's surface. It is not uncommon, during the process of producing the hydrocarbons, for gas or other production fluids to accumulate in the annulus below the packer. If the amount of gas or other production fluids accumulated in the annulus below the packer becomes too excessive, the gas or other fluids can interfere with the production process. Accordingly, in those situations, it becomes desirable to be able to seal the wellbore so that fluids from the wellbore cannot pass through the wellbore to the surface.
To seal the wellbore against unwanted pressurized fluids from reaching the surface. Typically a safety valve is placed in the wellbore at some point where the operator desires to block unwanted pressurized fluid flow from reaching the surface. Typically the safety valve is a spring operated valve that is held open by a control line that provides hydraulic pressure from the surface. In the event of an emergency or other well condition where the safety valve must be actuated the control line pressure is removed and the spring will close the safety valve.
There has been a recent trend in the oil and gas industry whereby companies have extended their exploration efforts into greater and greater depths. This is especially true as it relates to exploration companies venturing into water depths greater than approximately 2,000 to 3,000 feet to drill for oil and gas, such as in the Gulf of Mexico. As a result of this trend, a problem has arisen with the operation of the safety valves of the type described above. The problem relates to the means by which the safety valve is actuated, namely, by connecting a control conduit from the earth's surface to the annulus vent valve, and then applying hydraulic fluid through the control conduit to a piston within the safety valve to overcome the force of a return spring and move a flow tube to open and close a closure member, such as a flapper, which blocks or permits fluid flow from the secondary packer bore into the annulus above the packer. More particularly, the problem relates to the column of hydraulic fluid in the control conduit extending between the earth's surface and the safety valve; this is sometimes referred to as the “hydrostatic head”. Previously, it has been feasible to design a safety valve with a return spring capable of generating sufficient force to maintain the flapper in a closed position and overcome the force of the hydrostatic head. However, as wells are drilled to deeper and deeper depths, and as packers and safety valves are set at deeper and deeper depths, the length of the hydraulic control line necessarily increases, as does the force of the hydrostatic head. This requires the use of a larger and larger spring to overcome the increase in the hydrostatic head. It has been determined that, when a safety valve is set below certain depths, which is believed to be in the range of approximately 2,000 to 3,000 feet, it is no longer feasible to design a safety valve with a power spring large enough to overcome the hydrostatic head and close the safety valve in the case of an emergency.
Beyond this depth it has been preferred to use a balance line in conjunction with a spring in order to overcome the hydrostatic head in the control line and close the safety valve. However, typically a balance line may be run on the outside of the casing or other tubular and as such is susceptible to damage. In order to increase the operator's confidence that the safety valve has the capability to fail safely it is the object of this invention to provide a device that will allow the control line to automatically vent in the event of a balance line pressure failure.